Re: Xorg SEGV in Xen PV dom0 after updating from 5.16.18 to 5.17.5 #forregzbot

[Thorsten Leemhuis]

TWIMC: this mail is primarily send for documentation purposes and for
regzbot, my Linux kernel regression tracking bot. These mails usually
contain '#forregzbot' in the subject, to make them easy to spot and filter.

On 04.05.22 07:46, Thorsten Leemhuis wrote:
> On 04.05.22 02:37, Marek Marczykowski-Górecki wrote:
>>
>> After updating from 5.16.18 to 5.17.5 in Xen PV dom0, my Xorg started
>> crashing when displaying any window mapped from a guest (domU) system.
>> This is 100% reproducible.
>> The system is Qubes OS, and it uses a trick that maps windows content
>> from other guests using Xen grant tables, wrapped as "shared memory"
>> from Xorg point of view (so, the memory that Xorg mmaps is not just from
>> another process, but from another VM). That's the ShmPutImage you can
>> see on the stack trace below.
>> [...]
>> I don't see any related kernel or Xen messages at this time. Xorg's SEGV
>> handler prints also:
>>
>>     (EE) Segmentation fault at address 0x3c010
>>
>> Git bisect says it's bdd8b6c98239cad ("drm/i915: replace X86_FEATURE_PAT
>> with pat_enabled()"), and indeed with this commit reverted on top of
>> 5.17.5 everything works fine.
>>
>> I guess this part of dom0's boot dmesg may be relevant:
>>
>> [    0.000949] x86/PAT: MTRRs disabled, skipping PAT initialization too.
>> [    0.000953] x86/PAT: Configuration [0-7]: WB  WT  UC- UC  WC  WP  UC  UC  
>>
>> Originally reported at
>> https://github.com/QubesOS/qubes-issues/issues/7479
>>  
>> #regzbot introduced bdd8b6c98239cad
>> #regzbot monitor: https://github.com/QubesOS/qubes-issues/issues/7479

#regzbot fixed-by: 72cbc8f04fe2fa9